Blog: Shock protection

Date: 19 August 2015

Copyright © David Boettcher 2005 - 2023 all rights reserved.

I make additions and corrections to this web site frequently, but because they are buried somewhere on one of the pages the changes are not very noticeable, so I decided to create this blog section to highlight new material. Here below you will find part of one of the pages that I have either changed or added to significantly.

The section below about shock protection – what it is, who invented it and why it wasn't taken up more quickly than it was – is from my page about watch movements.

If you have any questions or comments, please don't hesitate to contact me via my Contact Me page.

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Shock Protection

The pivots of the balance staff of a watch are made very thin to minimise friction, and are therefore delicate. If a watch is dropped or knocked, the shock can cause the balance staff pivots to bend, or more usually to break because they are very hard and brittle, causing the watch to stop or run inaccurately. Pocket watches and early wristwatches were both subject to this problem. It was not so much of a problem for pocket watches, which were normally kept in a pocket and only received a shock if accidentally dropped, but wristwatches are in a vulnerable position at the end of the arm, where they are very prone to getting knocked. Broken balance staff pivots were a common occurrence in early wristwatches and every watch repairer kept extensive stocks of spare balance staffs. Today the situation is different: broken balance staffs are rare and most watch repairers cannot or will not replace a balance staff.

The picture here shows a cross section through the balance assembly of a watch. The gold coloured item is the balance, the thing you see oscillating backwards and forwards as the watch runs. The green coloured item is the shaft on which the balance turns, called the "balance staff". The red items are four "jewels" that provide hard and low friction surfaces for the staff to turn on. The two jewels that the pivots of the staff pass through are called jewel bearings or "jewel holes", and the two that the ends of the staff bear on are called "end stones" or "cap jewels". The pivots of the balance staff, the reduced diameter portions at the ends, are made very small in diameter, just a few hundredths of a millimetre, in order to minimise friction, and are hardened to minimise wear. They are therefore very delicate and dropping a watch or knocking it against a hard surface can cause them to break.

Broken balance staff pivots were a common occurrence in early wristwatches, and every watch repairer kept a stock of spare balance staffs. Expert watch repairers could also make a new balance staff on a lathe if necessary. Turning a round shaft on a lathe is not a difficult job in principle, but the very small size of balance staff, especially of a wristwatch, which is just a few millimetres long and with pivots only a few hundredths of a millimetre in diameter, makes turning a balance staff challenging.

To overcome the problem of broken balance staff pivots, shock protection systems were devised with the idea of isolating the pivots from shocks that affect the watch. In around 1790 Abraham Louis Breguet invented what was probably the first system, which he called the "parachute" or "suspension élastique". The endstones of the balance staff were held in place by spring blades so that they move in the event of a shock, cushioning the balance staff pivots. Breguet presented the definitive version of his design at the French national industrial exhibition of 1806.

Other shock protection systems followed over the years. The image here is from "The Wilderness Catalogue" of Robert Pringle & Sons from January 1931 and shows a shock absorber movement fitted to a number of watches in the catalogue. The movement was made by the General Watch Co. under the brand Helvetia. The shock protection shown in the image was granted a Swiss patent on 31 October 1930, patent No. CH143073, priority date 24 September 1929.

Today, when every modern mechanical watch has shock protection, it seems surprising that it took so long for shock protection to be adopted. The reasons for this, and the eventual universal adoption of shock protection, are an interesting story in their own right which I discuss in the section below called "slow adoption".

Incabloc

The best known and most widely used shock protection system was and still is Incabloc, invented by Fritz Marti.

Marti was a Swiss engineer working at Fabrique Election of La Chaux-de-Fonds, owned by Georges Braunschweig. Well aware of the weakness of balance staff pivots he determined to do something about it. In April 1928 he created a watch fitted with a shock absorbing system that used movable balance staff jewels to protect the balance staff pivots from shocks. This design was granted Swiss patent CH 141098, filed 27 July 1929 and registered 15 July 1930. In 1931 Georges Braunschweig and Fritz Marti established Porte-Echappement Universel SA, now better known as Portescap SA, the name it adopted in 1963.

At Porte-Echappement, Marti continued his work and on 2 March 1933 filed an application for a new patent for a simplified spring cushioned bearing system, which was granted as Swiss patent CH 168494 on 15 April 1934. Previous systems used two separate springs, one for lateral and the other for axial shocks. Marti's new design incorporated a cone shape for the bearing housing that redirected lateral shock in an axial direction, thus requiring only one spring. Production started in June 1933 and the Incabloc trademark was registered by the company in Switzerland on 6 July 1933.

All shock protection systems work in basically the same way. The balance staff jewel bearings are held in place by light springs instead of being mounted rigidly in the plates. When a shock occurs the springs allow the jewels and pivots to move slightly, and a stronger part of the balance staff contacts a fixed part of the housing to take the shock. Once the shock is is over the springs return the jewels and pivots to their correct positions.

The figure from the patent reproduced here shows this in action. In Fig 3 the balance staff has received a lateral impulse shown by arrow A. The staff pivot and its jewel bearing have mode to the right in the housing, and the cone shape of the jewel setting has caused it to ride up in the housing, lifting the cap jewel 5 against the spring 6. The stronger part of the balance staff 9 has contacted the housing to absorb the shock. In Fig 4 the balance staff has received an axial impulse shown by arrow B. In this case the jewel bearing and cap jewel have both been lifted vertically against the spring and the shoulder of the balance staff 10 has contacted the housing to absorb the shock.

The second figure reproduced here is from an Incabloc advert in 1953. It illustrates well all the components of the fully developed Incabloc design, including the famous “ lyre shaped spring”. This is the earliest advertisement mentioning the lyre shaped spring that I have found.

In the figure from the original patent, the spring labelled 6 which holds the cap jewel in place is shown fixed to the outer housing by a screw. Some time later, Incabloc engineers realised that they could make the spring clip into the housing and designed the lyre shaped spring, named after the musical instrument of similar shape. When looking at a watch with the fully developed form of Incabloc shock protection, the lyre shaped spring is clearly visible on the balance cock and its unique shape is a sure and easy way to identify an Incabloc shock proof setting.

The “T” shaped projection of the Lyre shaped spring clips into a slot in the housing and forms a type of hinge. The two opposite ends are sprung into a slot on the opposite side of the housing to retain the cap jewel and bushing in place. This makes it very quick and easy to service an Incabloc setting. Tweezers are used to spring the ends of the Lyre shaped spring out of place. The spring then hinges up and the cap jewel and bushing can be lifted out. This makes servicing an Incabloc setting much quicker than a traditional setting where the end stones are held in place with screws, oversoming one of the initial objections to shock proof settings.

Two of the first companies to adopt Incabloc were The West End Watch Co. in 1934 for their Sowar Prima, which became West End's most successful model, and Mido in 1935 for the Mido Multifort, became the best selling Mido watch until the 1950s. Both companies were also early adopters of the Taubert / Borgel company's waterproof Decagonal case.

Slow Adoption

Incabloc provided a real and commercial solution to the problem of broken balance staff pivots, but it wasn't immediately widely adopted. In fact, Incabloc was only one of many shock protection systems that had been developed since Breguet invented his "parachute" in 1790, but for some reason watch manufacturers were reluctant to incorporate these into their designs. The reasons for this, and the eventual universal adoption of shock protection, are an interesting story in their own right.

Shock protection for the balance staff pivots added extra cost, and most manufacturers were very cost sensitive, knowing that a higher price point would cost sales. There was also no demand at the time from the public for shock protection; everyone knew that watches were delicate and that if you dropped them they were liable to break, every watch repairer at the time was very well used to replacing balance staffs, and spare balance staffs were readily available from the manufacturers.

It might be thought that the military would be the first to require that shock protection be fitted. A watch with broken balance staff pivots is useless, and if that happened in the middle of a military manoeuvre it could be dashed inconvenient. One would think that army types would be breaking balance staffs left right and centre, but in fact the military were among the most reluctant to see watches fitted with shock protection.

The British "Army Trade Pattern" A.T.P and "General Services Trade Pattern" G.S.T.P. specifications emerged after the Great War and governed all British Army watches used before and during World War II, until they were superseded by the British War Office Specification No. R.S./Prov/4373A "Watch, Wristlet, Waterproof" or "W.W.W" specification in 1945. Neither the A.T.P, G.S.T.P. or the original 1945 version of the W.W.W. specification required shock protection, even though by the time of the W.W.W. specification shock protection of various forms had been commercially available for many years.

It is clear from this that breakage of balance staffs was not a great inconvenience to the British military. In fact, there was a dispute in the military committee that drew up the W.W.W specification about whether the benefit in reduction broken balance staffs result from shock protection would be offset by the increased time required for regular service of watches with shock proof settings, estimated at 10 - 15 minutes per watch. This was overcome by changes to the Incabloc design, the introduction of the clip in Lyre shaped spring, which made it quicker and easier to service a shock resistant setting than an old style setting with end stones held in place by screws. Nevertheless, it was not until 1947 that an amendment to the W.W.W. specification introduced a requirement for shock protection.

The conclusion must be that the general public and the military types who actually needed a wristwatch to perform their duties (mainly officers, not squaddies who were jumping in and out of trenches etc.) took care of them and that broken balance staffs were relatively rare. Before the second world war, vigorous pursuits such as trekking, diving and mountain climbing were the realm of a few and not followed as widely as they are today. If people outside the watchmaking profession even realised that shock protection was possible, they probably didn't think that they needed it.

The Power of Advertising

Shock protection was initially more widely advertised in America than in Europe. It seems that in America, with its more developed advertising industry, shock protection was used as an extra selling point or feature that could be used in advertising, persuading consumers that they needed it and to pay more for a watch with shock protection. Once watches that were "shock proof" entered the public's consciousness, the early adopters of such watches acquired "bragging rights", and any watch that didn't have shock protection was deemed old fashioned.

The feature that made Incabloc a commercial success, and the system that within a short period dominated the market for shock proof settings, was that it was a very clever modular design, making it easy for manufacturers to add to their movements, and it also speeded up watch servicing.

All balance staffs had, and still have, the same four jewels; two bearing or "jewel holes", and two "end stones" or "cap jewels". These all need to be cleaned when the watch is serviced, which means removing the cap jewels. In a watch without shock protection the cap jewels are held in place by one or two small screws, and the first shock protection systems used screws to hold the shock absorbing spring in place. Incabloc's breakthrough idea was to hold the cap jewel in place with the, now instantly recognisable, shaped Lyre shaped spring clip, which was also the spring that allowed the jewel to move when a shock occurred. Instead of taking longer to service than a setting without shock protection, Incabloc was faster, because instead of fiddling about with tiny screws, simply releasing the Incabloc spring, which was cleverly also held captive to the setting, allows the cap jewel to be removed, and then quickly clipped back into place after cleaning and oiling. This additional benefit was soon recognised and Incabloc came to dominate the market for shock proof settings.

The lifetime of Swiss patents in the 1920s and 1930s was normally 15 years, so Marti's patent for the original Incabloc system would have expired in 1949. After that many other shock protection systems such as Kif followed.

Downsides?

Shock protection is a real benefit, and today owners don't have to treat their watches as if they are delicate. A modern mechanical watch can be bashed about with gusto, and some owners take pride in wearing a watch that shows scars from trekking, mountaineering or diving. The only downside is for collectors of watches that were made before shock protection was widely used. The days of the local watch repairer who could replace a balance staff are now long gone, and watch repairers who can do this are few and far between, which makes replacing a staff with broken pivots an expensive operation. So if you have a watch without shock protection, take care of it!